(a) Field of the Invention
The present invention relates to a cata-dioptric type telephoto lens system and, more particularly, to a cata-dioptric type telephoto lens system utilizing a cata-dioptric system which is to be used with a single-lens reflex camera.
(b) Description of the Prior Art
Compared with photographic telephoto lens systems in general, a cata-dioptric type telephoto lens system can be designed to be extremely compact and light in weight and, moreover, has large merit that chromatic aberration to be caused by the detrimental secondary spectrum is very small. Therefore, various kinds of cata-dioptric type telephoto lens systems have been invented.
A known example of such cata-dioptric type telephoto lens systems is disclosed in Japanese published examined patent application No. 1909/72. The above-mentioned known example seems to be a lens system with f=500 mm considering from the field angle of its embodiment. Compared with general photographic telephoto lens systems with f=500 mm, the above-mentioned lens system is considerably compact and shows favourable characteristic of cata-dioptric type telephoto lens systems. However, the above-mentioned lens system is not yet compact enough.
Another example of known cata-dioptric type telephoto lens systems is shown in Japanese published unexamined patent application No. 131835/78. This example is an extremely compact lens system with a very small telephoto ratio, i.e., 0.29.
For cata-dioptric type telephoto lens systems, the focusing method to advance the front lens is generally adopted in order to focus the lens system easily. In case of this focusing method, the value of spherical aberration fluctuates remarkably when the distance to the object to be photographed changes. Therefore, even when aberrations are favourably corrected in respect to an object at the infinite distance, aberrations especially spherical aberration and coma are aggravated when photographing an object at a short distance. In case of the above-mentioned lens system shown in Japanese published unexamined patent application No. 131835/78, aberrations are not yet corrected really favourably from the above-mentioned viewpoint and, therefore, this lens system is not fully satisfactory for practical use. In case of the above-mentioned lens system according to Japanese published unexamined patent application No. 131836/78, it is tried to eliminate detrimental rays by arranging a lens L.sub.5 with a very large thickness between a lens L.sub.1 located on the object side and primary mirror L.sub.2 as shown in FIG. 1 and by adopting such method to apply black paint to the circumferential surface A of the thick lens L.sub.5. As, however, the lens L.sub.5 with a very large thickness is used, the lens system as a whole cannot be made satisfactorily light in weight. Besides, as it is evident from FIG. 1, the ray which enters through the lens L.sub.1 but which is not eliminated by the circumferential surface A of the thick lens L.sub.5 and enters the thick lens L.sub.5 through its surface R on the object side (for example, the ray l in FIG. 1) is refracted by the surface R and is directed to the direction shown by reference symbol l'. If the surface R exists at a position nearer the image side, for example, at the position R', the ray l is refracted by the surface R' and is directed to the direction shown by reference symbol l" because the surface R is a diverging surface. That is, the ray l" goes to a position out of the image surface. In other words, when the surface R is nearer the image side (when the lens L.sub.5 is thinner), the ray l goes to a position out of the image surface and does not cause a ghost. Therefore, it is preferable to make the lens L.sub.5 thinner also from this point of view.